Expand link checks to all types of external documents

[tidoust]

When it analyses document links to search for missing and inconsistent references, Reffy currently only extracts links to what it identifies as "friendly" specs (W3C and WHATWG specs, typically).

However, there is no a priori reason to do that: in theory, any link to an external document within a spec should warrant the creation of a corresponding entry in the References section.